This invention relates to the field of voltage controlled oscillator circuits.
Advances in communications systems have brought on a large increase in the demand for communications devices such as mobile/cellular telephones and radios, portable digital telecommunications and data devices, and the like. Typically such devices operate at a frequency within a band of the electromagnetic spectrum from about 900 MHz to about 2.5 GHz. For example, a cellular mobile telephone may transmit and receive signals at about 1800 MHz, and in order to do so the radio circuitry requires an oscillator tuned to that frequency. Other types of devices of course operate at different frequencies and therefore require an oscillator circuit tuned to a different frequency.
Voltage controlled oscillator (VCO) circuits are known which allow the oscillation frequency thereof to be controllably tuned through use of a control voltage. Known controllable oscillator circuits are useful in devices that operate over a range of frequencies or at a plurality of different frequencies, since only a single oscillator circuit is necessary. However, the frequency range of current VCO circuits is somewhat limited and such circuits are incapable of spanning the entire frequency band used for present and proposed applications such as those mentioned above. Some limitations to the controllable frequency range of known VCO circuits are inherent in the circuit structure or components and others are imposed by the desired application. For example, many of the applications for VCO circuits are in battery powered mobile devices which require low power consumption and operate at low supply voltages. This compounds the restricted range of known VCO circuits since the available range of control voltages that may be applied to the circuit to adjust the frequency is limited.
It is therefore desirable to provide a voltage controlled oscillator circuit that is capable of operating over a wide band of frequencies, or over a plurality of frequency bands, and which may operate using a relatively low supply voltage so as to be useful for battery powered applications.
In accordance with the principles of the present invention, there is provided a variable frequency signal generator having a pair of LC oscillator circuits interconnected by a transconductance control circuit. The variable frequency signal generator has two independently controllable inputs, and the two oscillator circuits provide respective quadrature output frequency signals.
A first control input is coupled to the oscillator circuits, and is preferably used to control the capacitance of varactors included in the oscillator circuits. In the preferred mode of operation, the first control input receives a first input voltage selected from first and second voltage levels corresponding to varactor capacitance values toward the respective upper and lower extremes of the capacitance range of the varactors. For example, the first voltage level applied at the first control input may drive the varactors into a strong inversion mode operation, and the second voltage level may correspond to a depletion mode operation.
A second control input is coupled to the transconductance control circuit, and is preferably used to control the amount of cross-coupling between the oscillator circuits, which varies the output frequency of the oscillator circuits. In the preferred mode of operation, the second control input receives a second control voltage that can be selected from within a voltage range. In the preferred embodiment, with the first voltage level applied at the first control input, the second control voltage can control the oscillator output frequency between about 840 MHz and 1.37 GHz. With the second voltage level applied at the first control input, the second control voltage can effect variation of the oscillator output frequency between about 1.30 GHz and 2.42 GHz.
In a preferred form of the invention, the variable frequency signal generator also includes a control voltage compensation circuit coupled between the first and second control inputs. The control voltage compensation circuit is controlled by the voltage on the second control input to regulate a bias voltage across the oscillator circuit varactors such that, with a constant voltage at the first control input, the bias voltage across the varactors is maintained substantially constant. This allows the varactors to operate at a fixed bias voltage, and thus maintain a substantially fixed capacitance, despite voltage level variations at the second control input.
In one form of the invention, the variable frequency signal generator may be characterized by:
first and second LC oscillator circuits including voltage controlled capacitance elements;
a first control voltage input coupled to the LC oscillator circuits to control the capacitance of the voltage controlled capacitance elements;
a transconductance control circuit cross-coupling the first and second LC oscillator circuits;
a second control voltage input coupled to the transconductance control circuit to control transconductance coupling between the first and second oscillator circuits; and
a variable frequency signal output coupled to generate an output signal from one of the first and second oscillator circuits, wherein the frequency of the output signal is controllable by independently controlling the voltages at the first and second control voltage inputs.
Preferably, each of the first and second LC oscillator circuits comprise a pair of transistors cross-coupled to first and second common nodes; and a respective LC load coupled to each of the first and second common nodes, each LC load comprising an inductive element coupled between the common node and a supply voltage and a varactor capacitive element coupled between the common node and a controlled voltage node. The first and second common nodes provide input/output connections for coupling to the transconductance control circuit and the variable frequency signal output.
Preferably, the transconductance control circuit comprises first and second coupling circuits, each coupling circuit having a pair of inputs and a pair of outputs. The coupling circuits cross-coupled the oscillator circuits, such that:
the inputs of the first coupling circuit are coupled to the common nodes of the first oscillator circuit, and the outputs of the first coupling circuit are coupled to the common nodes of the second oscillator circuit; and
the inputs of the second coupling circuit are coupled to the common nodes of the second oscillator circuit, and the outputs of the second coupling circuit are coupled to the common nodes of the first oscillator circuit.
In the preferred form of the invention each of the coupling circuits comprises a pair of coupling transistors having first conduction nodes forming the coupling circuit outputs and second conduction nodes coupled to a supply voltage through a control transistor, the coupling transistors having control nodes forming said coupling circuit inputs, and the control transistor having a control node coupled to the second control voltage input.
In the preferred embodiment, the control voltage compensation circuit includes a resistive element that provides a controlled voltage drop between the first control voltage input and the controlled voltage node so as to maintain the potential difference between the common nodes of said oscillator circuits and the controlled voltage node substantially constant with respect to changes in voltage at the second control voltage input.
In accordance with the present invention, there is also provided a method of operating a variable frequency signal generator having a pair of LC oscillator circuits interconnected by a transconductance control circuit, the oscillator circuits having outputs that in use provide respective quadrature frequency output signals, the oscillator circuits having a first control voltage input and the transconductance control circuit having a second control voltage input. The method comprises: applying a first control voltage to the oscillator circuits through said first control voltage input, the first control voltage being selected from first and second predetermined voltage levels corresponding to first and second output signal frequency bands; and applying a second control voltage to the transconductance control circuit through said second voltage input, the second control voltage being within a predetermined voltage range corresponding to an output signal frequencies within the first and second frequency bands. Preferably the oscillator circuits include MOSFET varactors, wherein the first and second predetermined voltage levels correspond to the MOSFET varactors operating in respective strong inversion and depletion modes.